Vehicle heating, cooling and ventilation systems incorporate a blower fan to force air across various heat exchangers to heat or cool the air, or may intake untreated air, after which it is forced into a duct system for distribution to the vehicle interior. Upstream of the fan, inlet air is drawn into the system, either fresh air from outside of the vehicle, or recirculated air from inside the vehicle. When the operator selects between fresh or recirculated inlet air, a flapper type valve door pivoted inside an inlet duct switches to block either an outside opening (generally located under a screen at the lower edge of the windshield) or an inside opening inlet (generally located under the vehicle interior instrument panel).
Typically, air inlets to the fan have simply drawn air directly into the system, whether from outside or inside, without an interposed intake filter. With an increasing emphasis on interior comfort options such as multi zone temperatures and individual driver-passenger controls, it has been proposed to filter the inlet air, just as combustion air has always been filtered. Patents in the area of inlet air filtration often disclose filters located under the vehicle hood, accessible from outside the vehicle, upstream of the fan. An example may be seen in U.S. Pat. No. 4,779,468. Such designs act to filter only outside air. Others show a filter located more or less in a flat plane, just downstream of the fan, so that air drawn in by the fan, through whatever inlet opening, is filtered. An example may be seen in U.S. Pat. No. 4,925,468, FIG. 2, where a flat filter 120 inserted into a slot 108 in duct 103, downstream of fan 106, filters air pulled by fan 106 through either fresh air inlet 104 or recirculated inlet 105, as selected by flapper door 102. A drawback of such a design is that the space available for insertion of the flat filter into the duct may often be limited, requiring that the filter be broken up into two or more hinged sections, so as to be installed section by section. Even if access to the system were better at a location upstream of the blower 106, nearer to the two inlets 104 and 105, interference by the flapper door 102 would prevent the location of a filter inboard of the inlets 104 and 105.